[delphizip] Multithreading
- From: "James Turner" <james.d.h.turner@xxxxxxxxxxxx>
- To: <delphizip@xxxxxxxxxxxxx>
- Date: Tue, 19 Nov 2002 22:50:45 -0000
Reading through the discussion on this subject, I'm inclined towards the view
that some people are unclear as to the benefits of multithreading.
The only advantage to be gained from having a Zip/Unzip DLL that is thread-safe
is that it can be safely called simultaneously from two or more threads in a
single process. As Eric said previously, very few people are likely to require
this.
You can use the standard DLLs within a multithreaded program provided only one
thread ever accesses the DLL at a time (typically the main thread).
Using thread-safe DLLs will in no way make a program that uses a single thread
more responsive (unless something very clever has been added).
If a program appears to become unresponsive whilst the DLLs are active it is
because the DLLs are failing to check for and dispatch messages (something they
probably should do if called from the process's main thread). However, there is
no point checking for messages unless the DLL also checks for such things as an
abort flag being set in response to a button click.
Of course, failing to check for messages during any long operation will cause a
program to become unresponsive - it makes no difference if the relevant program
code is in a DLL or in the main executable program.
In a Delphi application, checking for messages involves periodically* calling
Application.ProcessMessages.
In a DLL, the best that can be done (so far as I am aware) is to periodically*
call a procedure like the one below.
procedure CheckMessages;
var Msg : TMsg;
begin
while PeekMessage(Msg,0,0,0,PM_REMOVE) do begin
TranslateMessage(Msg);
DispatchMessage (Msg);
end;
end;
*In the context of zip/unzip operations, periodically might mean on every
buffer flush operation. Also, you can check for messages in response to the
OnProgress event.
I hope this clears up (what seems to be) some confusion.
-- James Turner
www.skaro.net
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